1. Field of the Invention
This invention relates to a novel granular or powdery phenol-formaldehyde resin and a process for its production. More specifically, it relates to a novel granular or powdery phenol-formaldehyde resin which has reactivity and good storage stability and flow characteristics and is suitable as a molding material, and to a novel process for its production.
2. Description of the Prior Art
Typical known phenol-formaldehyde resins are novolak resins and resol resins.
The novolak resins are usually produced by reacting an excess of phenol with formaldehyde in the presence of an acid catalyst such as oxalic acid (usually in an amount of 0.2 to 2%) while maintaining the mole ratio of phenol to formaldehyde at, for example, 1:0.7-0.9. The novolak resins so produced have no self-crosslinkability and are thermoplastic because they are composed of, as main components, tri-, tetra- and pentamers resulting from the bonding of phenol moieties mainly by methylene groups and contain almost no methylol groups. The novolak resins can be converted to cured resins by, for example, reacting them under heat with a crosslinking agent, such as hexamine (hexamethylenetetramine), which is at once a formaldehyde generator and an organic base (catalyst) generator, or by mixing them with a solid acid catalyst and paraformaldehyde and reacting them under heat. When such a novolak resin in accordance with the former method is used as a molding material, the resulting molded article will be foamed owing to the generation of ammonia by the decomposition of hexamine or the undecomposed part of hexamine, or an organic base formed as a by-product will remain in the molded article. This causes the defect that the properties of the molded article are deteriorated, and the curing reaction is time-consuming. According to the latter curing method, those parts of the novolak resin which make contact with the paraformaldehyde and the acid catalyst undergo excessive crosslinking reaction, and it is difficult to cure the resin uniformly. Furthermore, the acid catalyst or paraformaldehyde remains in the molded article to degrade its properties with the lapse of time, or troubles such as foaming occur owing to the decomposition of the acid catalyst or paraformaldehyde during curing. Another defect is that when the novolak resin is to be mixed with another resin, hexamine, the acid catalyst, paraformaldehyde, etc. remaining in it deteriorate the other resin, and that the novolak resin contains a relatively large amount (for example, about 0.5 to 2% by weight) of free phenol because of the use of an excess of phenol as a starting material.
A process for producing cured novolak resin fibers was recently suggested which comprises heating a novolak resin at a high temperature to form a product having a considerably high degree of condensation, purifying the product by removing components having a low degree of condensation, thereby to obtain a product having a relatively high degree of condensation and comprising phenol moieties linked to each other by 7 to 10 methylene groups, melt-spinning the product to form novolak fibers, dipping the fibers in an aqueous solution of hydrochloric acid and formaldehyde and gradually heating the solution from room temperature to allow curing reaction to proceed from the surface of the fibers (Japanese Patent Publication No. 11284/1973). This process requires an extra step for the formation of the novolak resin having a high degree of condensation. Furthermore, when this resin is pulverized without being converted to fibers, and subjected to an external curing treatment, it is almost impossible to allow the curing reaction to proceed uniformly to the interior of the resin. Granules or powders obtained by cutting or pulverizing the cured fibers are expensive, and do not possess good flow characteristics.
On the other hand, the known resol resins are produced usually by reacting phenol with an excess of formaldehyde in the presence of a basic catalyst (about 0.2 to 2%) such as sodium hydroxide, ammonia or an organic amine while maintaining the mole ratio of phenol to formaldehyde at, for example, 1:1-2. The resol resins so produced contain mono-, di- and trimers of phenol having a relatively large amount of methylol groups as main components and are very reactive. It is the usual practice therefore to store them in a refrigerator as a water or methanol solution having a solids concentration of not more than 60%. The period for which such storage is possible is about 3 to 4 months at the longest. To mold and cure such a resol resin, the water or methanol is removed and the resin is heated in the optional presence of an acid catalyst. The rate of this curing reaction is very high, and, for example at 150.degree. C., gellation occurs within several tens of seconds.
Since the resol resin has very high reactivity, it cannot be obtained as a stable granular or powdery solid. Furthermore, because a cured product of the resol resin has a highly developed three-dimensional structure, it is very hard and its conversion to a fine granular or powdery molding material is quite difficult (Japanese Patent Publication No. 12958/1978).
Several years ago, a process was disclosed which comprises reacting a phenol and formaldehyde in the presence of at least a nitrogen-containing compound, and reacting the resulting condensate with a hydrophilic polymeric compound to form a granular or powdery resin (Japanese Patent Publication No. 42077/1978). The resulting resin in the nongelled state contains as much as about 5 to 6% of free phenol (Examples 1 to 4 of the cited Japanese patent document), and a gelled product of the resin (Example 5 of the Japanese patent document) is a very hard non-reactive resin. Molded articles obtained from the gelled resin have deteriorated properties because of its inclusion of the nitrogen-containing compound used as catalyst or the hydrophilic polymeric compound.
A process is also known which comprises reacting a phenol and formaldehyde in a basic aqueous solution, mixing the resulting prepolymer with a protective colloid, and coagulating the prepolymer under acidity to form inert solid beads (Japanese Patent Publication No. 13491/1976). The coagulated product corresponds to a cured product of a resol resin, and has no reactivity. Furthermore, since it contains a salt or acid and the protective colloid, molded articles prepared from it have degraded properties.